This section provides background information related to the present disclosure which is not necessarily prior art.
The technology of the offshore oil/gas production currently faces two major challenges: 1) design of a floating production facility, which has good hydrodynamic performance and can be used to store and offload oil in deep water and adopted with drilling and dry-well facilities; and 2) utilization of associated gas of oil field and development of natural gas fields, which are located in deep water and far from the shore.
Regarding the first challenge, good solutions have been found, such as TLP (Tension Leg Platform), SPAR, and SEMI (Semi-submersible Platform), all of which have good hydrodynamic performance and are floating platforms able to be adopted with drilling and used in deep water. Although the ship-shaped FPSO (Floating Production, Storage and Offloading) has worse hydrodynamic performance than the three platforms mentioned above, it still can satisfy the requirement of offshore oil production and storage and be utilized in deep water even with bad conditions. Therefore FPSO has become the main type of the offshore oil production facility. Another system, FPDSO, which further adopts drilling machinery above its moon pool of the FPSO, has been researched for several years. Furthermore, about ten years ago, SBM Offshore invented the tension leg deck (TLD) to handle the challenge of large heave motions, which bring in difficulties in drilling operations and in preventing the use of dry-wells. The FPDSO can only be used in good sea conditions, due to the limitation of the ship-shaped FPDSO design. The solution for the first challenge has been provided in China patent application No. CN 200810024562.4, entitled “Floating Platform With Underwater Liquid Storage,” and filed by Zhirong Wu.
However, regarding the second challenge, people have not found satisfying solutions. Compared to the development of oil fields, the development of gas fields, which relates to the storage and transportation of the natural gas, is especially difficult. The traditional solution is to use subsea pipelines to transport natural gas produced offshore to onshore users directly. The onshore users can use natural gas as fuel or chemical raw materials, or liquefy natural gas for the same use. However, it is not always economic to build subsea pipelines, when the amount of gas produced is small, the gas field is far from users, or the users lack related infrastructure. Furthermore, the utilization of associated gas is also a challenging process. Burning out the associated gas is banned by law now. However, pressing the natural gas back to the ground is costly and is not workable for all kinds of geology.
Currently, the storage and transportation of offshore natural gas often adopts the following four methods. 1. GTL (gas to liquid): synthesize natural gas to liquid hydrocarbons or methanol, and then export them by transport ships. 2. LNG: liquefy natural gas at low temperature, such as −162° C., and then export LNG by transport ships. 3. CNG (compressed natural gas): compress natural gas to some pressure, such as several ten bars or 150 bars, store it in a steel storage container, and then export it by transport ships. 4. GTW (gas to wire): use natural gas to generate electricity offshore, and then export the electricity through the subsea cables. In conclusion, the research directions lead to two aspects: 1) fixed and floating topside production facility for utilizing or converting the natural gas, which needs more efficient production processes and recovery of the natural gas, and 2) the lower base structure, especially for the floating structure, which is not only the structural basis of the topside facility, but a liquid storage tank. If it is designed for being used in a floating condition, it shall provide enough size for an adequate deck area and good hydrodynamic performance. Furthermore, the production facility shall be reliable, safe, friendly to the environment, reusable, and cost-effective.
In the last ten years, people have put a lot of efforts into the research of the production processes of GTL and LNG. LNG has become 7% of the global natural gas consumption in 2007. The demand of LNG is increasing 8˜10% each year. The demand of LNG is estimated to be 5 hundred million tons in 2030. Large market demand and challenges of development of the offshore natural gas have made oil companies and engineering/construction companies invest a large amount of resources to develop floating liquefied natural gas (FLNG) and floating oil and natural gas (FLOG) systems.
However, the research in lower base structure, especially for a floating structure, still has not made any significant progress. TLP, SPAR, and SEMI usually have limited sizes of the deck area and no storage function. Although the FPDSO and FPSO have larger water plane area, the draft is not deep (usually ˜20 m). Because of the vessel's larger water plane area and shallow draft, the heave motion, the pitch motion, and the roll motion of the FPSO/FPDSO system are large. Large heave motion is not good for installing drilling and dry-well in a floating structure. Although TLD can improve the heave motions of the wellhead, the problem of the heave motion of the overall floating structure still exists. Large heave, pitch, and roll motions of the floating structure also have a negative impact on the reliability of the entire system and cause fatigue damages. The problem of the instability of the existing floating structure is the reason why the synthesis of the natural gas to the liquid hydrocarbons or methanol offshore has not yet achieved any significant progress. If people can put the LNG storage underwater, where there is little wave effect, and optimize the structural configuration, such as to decrease the size of the water plane of the floating structure, the hydrodynamic performance of the structure can be improved. Therefore, a process of storage of LNG and LPG underwater becomes necessary. The solution for offshore conventional liquid storage and transportation at normal temperature and pressure has been provided in China patent application No. CN 200810024564.3, entitled “Underwater Liquid Storage, Loading, and Offloading System,” and filed by Zhirong Wu. However, this solution can not be used in liquid storage and transportation at abnormal temperature or pressure, such as the LNG or LPG storage and transportation.